A Taliban suicide bomber managed to find his way into a U.S. military base in Kabul, Afghanistan, where he detonated a bomb and killed four Americans. The blast left 16 wounded:

The suicide bomber at Bagram Air Field had been dressed as a day laborer and detonated the explosives in the vicinity of a dining facility around 5:30 a.m., according to a foreign security source. The sprawling base, which contains tens of thousands of contractors, is often targeted by Taliban rockets and attacks on patrols near the base, but suicide bombers hadn’t previously succeeded in breaching the outer layers of security.

U.S. Secretary of Defense Ash Carter said the bomb killed two service members and two contractors:

Carter offered his condolences to the families of those killed and said the injured were getting the best possible care.

“Force protection is always a top priority for us in Afghanistan, and we will investigate this tragedy to determine any steps we can take to improve it,” he said.

“For those who carried out this attack, my message is simple. We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future.”

The 16 wounded included U.S. service members and a “Polish soldier participating in a NATO mission.”

NATO Secretary General Jens Stoltenberg reiterated the organization’s commitment to fight terrorism:

“NATO remains committed to our mission to train and support the Afghan forces. We all stand united in the fight against terrorism,” he said in a statement.

Taliban spokesman Zabihullah Mujahid confirmed the terrorist group performed the attack and they planned it for four months.

Now the Pentagon must confront security issues. As mentioned before, officials kept the base heavily guarded and yet the bomber found his way inside. CNN Pentagon correspondent Barbara Starr addressed the issue:

“The question now is, how did someone get inside?” she said. “There’s very strict security at these bases. … If you come in a vehicle, you are stopped well before the outside gate. Vehicles are searched, people are searched, people must have ID — so it’s very difficult to understand right now how this happened.”

There are about 14,000 people on the Bagram base, with about 60% contractors, according to Starr. Locals move on and off the base in some contractor roles, she said.

Security breaches at coalition bases in Afghanistan have occurred but are rare, she added.